The present invention relates to photolithography.
Photolithography is widely used to form patterns on semiconductor wafers in fabrication of integrated circuits. A wafer 110 (FIG. 1) is coated with a photoresist layer 120. Photoresist 120 is irradiated from a light source 130. A mask or reticle 140 is placed between source 130 and resist 120. Mask 140 carries a pattern consisting of opaque and clear features. This pattern defines which areas of resist 120 are exposed to the light from source 130. After the exposure, the resist 120 is developed so that some of the resist is removed to uncover the underlying substrate 110. If the resist is xe2x80x9cpositivexe2x80x9d, then the resist is removed where it was exposed to the light. If the resist is xe2x80x9cnegativexe2x80x9d, the resist is removed where it was not exposed. In either case, the remaining resist and the exposed (uncovered) areas of substrate 110 reproduce the pattern on mask 140. The wafer is then processed as desired (e.g. the exposed areas of substrate 110 can be etched, implanted with dopant, etc.).
The resist pattern on wafer 110 is not always a faithful reproduction of the mask. In FIG. 2, an opaque feature 210M on mask 140 has a concave corner 220M. Feature 210M should ideally be printed (reproduced) in resist 120 as feature 210R, with a corner 220R. In fact, the resist region 230 in the corner""s cavity gets underexposed. As a result, the corner is smoothened in the resist pattern, as shown by line 240. See U.S. Pat. No. 6,280,887 issued Aug. 28, 2001 to Lu.
The resist pattern can be corrected with a serif 310 (FIG. 3). The serif is a region cut out in opaque feature 210M to increase the exposure of region 230. However, if the feature 210M is narrow, i.e. the dimensions D1, D2 are small, the serif can be difficult to form on the mask.
The invention is defined by the appended claims which are incorporated into this section in their entirety. The rest of this section summarizes some features of the invention.
Some embodiments of the present invention provide alternative techniques to reduce underexposure of the resist. In some embodiments, the resist is exposed twice. One exposure is through a mask like in the prior art, for example, as in FIG. 2 or 3. The other exposure is conducted through a different (xe2x80x9cmodifiedxe2x80x9d) mask which exposes resist regions which correspond to opaque regions of the first mask. This xe2x80x9cmodifiedxe2x80x9d exposure is not conducted with a sufficient light energy dose to create a resist pattern on the wafer. For example, in the case of the positive resist, the modified exposure dose is insufficient to cause the resist to be removed during the developing process. However, the modified exposure increases the total energy dose delivered to regions such as 230. A more faithful pattern reproduction results in some cases.
The modified and non-modified exposures can be performed in any order.
In some embodiments, the modified exposure does not use a mask. The entire resist surface is exposed.
Other features and embodiments are described below.